This is a continuation-in-part of application Ser. No. 625,784 filed Oct. 24, 1975, now abandoned, which was a continuation of application Ser. No. 533,364 filed Dec. 16, 1974, now abandoned.
Magnetic cores, such as toroidal-shaped cores, have been previously used for temperature sensing. The prior art methods of temperature sensing utilized transition characteristics of the magnetic core such as the Curie temperature transition and/or first order transitions such as those described in U.S. Pat. No. 3,534,306, issued on Oct. 13, 1970, in the name of Watrous et al. Prior temperature sensing devices of this type relied on the fact that at a certain temperature a drastic change of the magnetic characteristics of the core would occur. Thus, if a wire were wound around the core to form an inductance element, the inductance of the element would change drastically when the predetermined temperature was reached. This required specific core materials that were specially formulated and carefully controlled in order to provide the desired rapid transition at the exact temperature that was desired. A different specially manufactured magnetic core would then have to be substituted in the sensor in order to sense another temperature.
The sensing device of the present invention, by contrast, does not depend upon any rapid change of inductance state of the magnetic core. In the present sensing device, the inductance of the cores varies in a gradual manner until the inductance of both cores is approximately equal at a predetermined temperature which is then sensed by the sensing circuit. The advantage of this approach over the prior art devices is that by changing the inductance of the core by changing the number of windings on the core, the crossover point where the two inductances are equal may be changed so that the temperature sensor may be used over wide range temperatures.
The present invention is achieved by coupling the two inductively wound cores having different inductance vs. temperature characteristics into a four-arm A.C. inductance bridge circuit having two terminals that are connected to a conventional null detector. When the inductance vs. temperature characteristics of the two cores cross at a predetermined temperature, the inductances are equal; and the null detector indicates that the desired temperature has been reached. Although two magnetic cores have been connected in series to achieve temperature compensation, as is shown in U.S. Pat. No. 3,824,502; issued on July 16, 1974, to Bardash et al, the utilization of two series connected magnetic cores that have different temperature characteristics for sensing temperatures over a relatively large range of temperatures without a transition change of the magnetic state of the core has not previously been accomplished.
The applicant's temperature sensor devices, consisting of the two independent inductively wound cores may be employed by placing the cores both in the same temperature environment. They also may be used with one of the cores, placed in a reference temperature environment and the other core placed in an environment to be sensed. In this mode of operation they are capable of functioning somewhat like a thermocouple. This capability is not found in the device of the Robbins U.S. Pat. No. 1,945,378 in which a unitary transformer structure has one arm made of a material in which the permeability varies with temperature. The device of the Robbins patent was massive and thermally slow.
Also, the device of the Robbins patent depends upon a division of flux between the relatively constant permeability vs. temperature arm and the variable permeability vs. temperature arm. Functionally, this is of significant importance since, when the Robbins device is used to indicate a temperature, both arms of the device will be substantially at the same temperature due to heat conduction of one arm to the other. This means that the device of the Robbins patent is essentially useful only when both of the arms are contained in the same temperature environment. On the other hand, a thermocouple will not operate if both of the elements are placed in the same temperature environment. Thus, unlike either the thermocouple or the device of the Robbins patent, the present invention may be utilized in both types of applications without modification of the sensor other than to provide sufficient lead length, because of the provision of two separate and independent magnetic cores having controlled temperature dependent permeability.
When the sensing device of the present invention is used, constant monitoring of the temperature difference between two separate temperature environments may be achieved, and with this type of sensing the fast thermal response and the small size of the magnetic cores of the present invention are often of real significance. These sensors alternately may be used in a single temperature environment by placing both of the cores in the environment and, thus, versatility is another important advantage of the present invention.